1. Field of the Invention
The present invention relates generally to a solar cell, and more particularly to a method for producing front and rear contacts on a solar cell.
2. Description of the Related Art
Highly efficient solar cells made of polycrystalline or monocrystalline semiconductor material, for example of silicon, have to be optimized in all details to give high efficiency. For this reason, use is made, for example, of wafers which have only weak doping and therefore have a low defect density. The pn transition is produced on the front or light-incident side, for example, by diffusing in phosphorus, with the highly doped zone thus created having only a small depth.
To improve the conducting away of current at the rear side of the solar cell, the doping is increased directly under the rear side contact. For this purpose, use is usually made of aluminum which is, for example, applied and diffused in on the rear side by vapor deposition or in another way.
Various methods and processes are known for metallizing the solar cells and for producing the contacts to conduct away the current. Structured metallization can be produced in a simple way by printing. A screen printing process using an electrically conductive paste containing metal particles can be used to produce any electrode structures. Metallization can be produced by vapor deposition of thin metal layers and be reinforced in a further step, for example by immersion soldering, electrolytic deposition or electroless chemical deposition. On appropriately prepared surfaces, it is also possible to produce the metallization by chemical deposition directly on the semiconductor surface.
Front side contacts applied by screen printing can only be produced in structural widths down to a minimum of about 80 .mu.m. To further increase the performance of the solar cells, finer structures are required for the front side contact, so as to reduce the shadowing of active solar cell surfaces caused by these.
WO-A-91 08 584 discloses a process for producing a point-contact solar cell in which a pattern of microscopic openings is produced for the front side contact by means of a laser. Point-shaped nickel contacts are produced in these openings, which contacts are reinforced and electrically connected to one another by chemical deposition of copper. For the rear side contact, use is made of an aluminium layer which is applied by vapor deposition or is printed on.
European Patent Application 15 63 66 discloses a process for producing a solar cell having front side contacts arranged in channels in which the channels are produced by laser engraving. The front side contacts can be produced by filling the channels with electrically conductive paste and subsequent firing of the same and be reinforced by immersion soldering. Alternatively, the front side contact is produced by electroless deposition of nickel, is sintered and reinforced by copper plating.
Solar Cells, Volume 15, No. 1, September 1985, pages 1 to 25, discloses a thick film technique for producing solar cell contacts. For the rear contact, use is made of a screen printing paste comprising silver and aluminum.
M. A. Green: "High efficiency silicon solar cells" discloses a solar cell having a textured front side and front side contact arranged in channels. Front and rear side contact can be produced by simultaneous electroless deposition of nickel and subsequently of copper.